Method for gas relief and blowing of digesters for cellulose



Aug. 15, 1961 B. s. NYQUIST 2,996,423

METHOD FOR GAS RELIEF AND BLOWING OF DIGESTERS FOR CELLULOSE Filed May14, 1957 INVENTOR B6RJE SUNE NYQUIST United The present inventionrelates to a method for gas relief and for the blowing of digesters forcellulose, particularly sulphate cellulose, which method ischaracterized in that the relief or a part thereof, is carried outthrough the blowing conduit leading to the diffuser, and that theblowing is carried out immediatelyafter a relief is carried out in thismanner.

The invention also relates to a device for carrying out the said method,which device consists of a pipe conduit which connects the top of thedigester, preferably at the gas relief screen, with the blowpipe of thedigester after the blowing valve of the digester.

In conventional practice for digesting sulphate pulp the finaltemperature is usually about 170 centigrades and the pressure is about 7atmospheres. On interrupting the digestion the pressure is reduced bygas relief from the digester to a pressure of 4-5 atmospheres,whereafter the digester contents are blown into a diifuser.

A plant for cooking and washing sulphate cellulose is showndiagrammatically in the accompanying drawing.

The gas relief is carried out from the top of a digester 1 through ascreen which top is connected to a trap 3 by means of a pipe conduit 2in which trap pulp and black liquor brought along are separated out.From the trap 3 the relief gas passes to a condenser 4 in which the heatof the relief steam is recovered in the form of hot water. At thebeginning of the reduction of the pressure the gas is comparatively dryand contains a small quantity of turpentine vapours. These are condensedand usually recovered from the condensate. When the relief has beencarried on for some time, the gas becomes nearly free of turpentinevapour, but by this time some pulp and black liquor are easily broughtalong, particularly if the reduction of pressure is carried out quickly.The pulp and the black liquor which are brought along to the trap oftencause inconveniences.

The blowing of the digester is usually carried out through a conduit 5,the blowing conduit, from the bottom of the digester through a centralcock 6 to which the blowing conduits of a number of digesters areconnected. The blowing continues through a pipe conduit 7, which isconnected to the central cock 6, and through a rotatable pipe 8 to adiffuser 9. In the diffuser 9 pulp and liquor are deposited, while thetemperature of this mixture is lowered through formation of steam onaccount of the reduction of pressure occurring. The steam which containsas much heat as corresponds to the heat liberated by the reduction ofpressure, is carried through the return portion of the rotatable pipe 8to a central trap 10, where remainders of liquor and pulp brought alongare separated, and from there to a condenser 11 where the heat iscollected in the form of hot water.

According to another embodiment the blowing is not carried out to adiffuser 9 but to the central trap 10 where the temperature of the pulpand liquor blown is lowered through formation of steam in the same wayas in the diifuser. Thereafter the steam which is carried to thecondenser system 11, and the pulp and the liquor are allowed to flowdown through a rotatable pipe to the diffuser which is provided with agas outlet directly to the atmosphere.

At the beginning of the blowing the system is filled with air which iscarried to the condenser 11 together with rates are" steam. Owing to thegreat quantity of incondensable gases the condenser "11 does not worksatisfactorily. The resistance to flow in the condenser 11 and in thefollowing pipe conduit 12 is therefore considerably greater than afterthe air has been driven out. Therefore the deposit of pulp and liquor inthe diffuser at the beginning, before the air has been driven out, needsto be carried out at a pressure amounting to about 2 atmospheres gauges,that is, corresponding to a temperature in the pulp and the liquor ofabout centigrade. When the air has been driven out and the condensationhas started satisfactorily, the pressure in the diffuser 9 decreases andthe pulp deposited at the beginning begins to boil. Due to the greatfoaming which easily occurs with regard to the alkaline liquor some ofthe pulp brought along is caused to leave with the steam, with thedisadvantages which are caused thereby.

In the mixture of pulp and liquor deposited at the beginning of theblowing some air is always admixed therewith. The effect of thisadmixing is that the digester contents are not held in the diffuser 9,in spite of the fact that the volume of the latter is considerablygreater than that of the pulp and the liquor.

Now it has been found that these disadvantages are eliminated if thewhole or at least the latter part of the gas relief is carried outthrough the blowing conduit 7, the diffuser 9 and the condenser system11 belonging thereto. This is brought about in a simple manner wherebythe relief conduit or the screen belonging thereto in the top of thedigester 1, is connected by a new pipe conduit 13 to the blowing conduit5 of the digester before the central cock 6.

According to the other embodiment, when the blowing is carried outdirectly to the central trap 10 it must be seen to that the resistanceto flowing in the condenser 11 and the diffuser 9 is so measured thatall air is driven out of both, if the relief is not conducted through aspecial conduit through the diffuser 9 and thereafter through thecentral trap 10 and the condenser 11.

The following advantages are gained through this method: No difficultieswill arise with pulp and liquor in the trap 3 and the condenser system 4for the relief. The relief can be carried out extremely rapidly withoutinconveniences, pulp and liquor brought along stopping in the diffuser9. Due to this rapid relief a constant time for pressure reduction canbe kept from one digestion to another, the result being a more uniformquality of pulp. By means of this gas relief through the blowing conduit7, the diffuser 9 and the condenser system 11 belonging thereto all airis driven out of these, and the blowing is greatly facilitated. Forexample, right from the beginning the blowing valve can be entirelyopen. Due to the fact that the great resistance to flowing in thecondenser 11 and the following pipe conduit 12 is eliminated no boilingwill take place in the pulp and liquor already deposited, and therebythe horizontal Stratification obtained in the diifuser 9 at a blowingsatisfactorily carried out, is not either destroyed. In this manner abetter washing of the diffuser in all respects is obtained.

Due to the fact that all air has been driven out before the blowingstarts, no air is admixed in the diffuser 9 and thereby the digestercontents will obtain greater room therein.

Since not the whole relief, but only the latter part thereof must beconducted through the blowing conduit 7, the diffuser 9 and thecondenser system 11, about the same amount of turpentine as before canbe extracted. Thus, the yield of by-products is not much smaller.

What I claim is:

In a method for gas relief and subsequent blowing of digesters forcellulose cooking wherein during the blowing operation the digestercontents of pulp and liquor is passed to a diffuser and condenser systemso as to collect said contents in the diffuser and to condense flashvapors thereof in the condenser, the step of prior to said blowingoperation passing relief gases from the digester through said system ofdiffuser and condenser too-drive out air from said system and replaoethe same such relief gases;

A 4 7 h References Cited in the file of this patent UNITED STATESPATENTS 1,327,666 Pagan Jan. 13, 1920 5 1,908,615 Oman May 9, 1933FOREIGN PATENTS 608,515 France Apr. 23, I926

